6 Answers

NO!!!, they may have caused structural damage and also breached the fire regulations as fire/carbon monoxide may enter the loft conversion. Heat may also buckle the steel rsj. Inform your local building control. Have the inspector call

Did they ask your permission in writing explaining the works they were doing?
By the rules of the party wall act 1996 they should send you a letter informing you of works! If memory serves they have to allow you 2 weeks to react or reply.

Appart from changing a socket or hanging a picture most works involving the Party Wall has to be sent letter form to the neighbour,whether its only going half way through the wall...They are totally at fault to break through into your property.

If you cant approach them to discuss then i would talk to trading standards for further advice.

no even if you sighed a agreement the surveyor would never agree to the steel going into your property.
8.30am tomorrow call the local council, building control and tell them what has happened. they should come out that day.even if you dont have a party wall aggreement you can go to court and get a injunction and seek compansation. but no steel should be in your property and never positioned on a chimney. take pictures, speak to building control and get their engineer come around, they have the power to stop the work if its unsafe.
the builder should repair any damage but you are intitled to get a builder of your own to repair the work and your neigbour will be responcable for the bill.
hope this helps if i can clairify anything for you please get in touch